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Abstract:

A medical implant device comprises a substrate (10) having an undulating
surface provided by peaks (12) which are separated by recesses (14). The
device includes a porous coating layer provided on the undulating surface
of the substrate which comprises a plurality of particles (16). The
spacing between adjacent peaks on the surface of the substrate is less
than the particle size of the particles. The particles are bonded to the
peaks on the surface of the substrate and adjacent particles are bonded
to one another.

Claims:

1. A medical implant device, comprising a substrate having an undulating
surface provided by peaks that are separated by recesses, wherein the
device includes a porous coating layer provided on the undulating surface
of the substrate and the porous coating layer includes a plurality of
particles, wherein the spacing between adjacent peaks on the surface of
the substrate is less than the particle size of the particles, and
wherein particles are bonded to at least some of the peaks on the surface
of the substrate and adjacent particles are bonded to one another.

2. The device of claim 1, wherein the peaks are spaced apart regularly on
the surface of the substrate.

3. The device of claim 1, wherein the peaks comprise ridges, each ridge
having a width and a length, the length being greater than the width, and
wherein the recesses comprise troughs.

4. The device of claim 1, wherein the recesses comprise troughs and
wherein the peaks are surrounded by the troughs.

5. The device of claim 1, wherein the surface of the substrate is
provided by a metal and wherein the particles are formed from a metal.

6. The device of claim 5, wherein the particles are bonded to the peaks
on the surface of the substrate and to one another by means of a
sintering process.

7. The device of claim 1, wherein the particles have an approximately
spherical shape.

8. The device of claim 1, wherein at least some of the recesses have a
base and wherein the bases between adjacent peaks are rounded.

9. The device of claim 8, wherein the portions of the bases of the
recesses that are rounded have an approximately constant radius.

10. (canceled)

11. The device of claim 1, wherein the peaks are rounded.

12. The device of claim 11, wherein the portions of the peaks that are
rounded have an approximately constant radius.

13. The device of claim 12, wherein the ratio of the radius of the peaks
to the size of the particles that are bonded to the peaks is not more
than about 0.7.

14. The device of claim 1, further comprising a cover layer that overlies
the porous coating layer and which includes a plurality of cover layer
particles, wherein the cover layer particles are bonded to the particles
of the coating layer and adjacent particles of the cover layer are bonded
to one another, and wherein the particles of the coating layer are
approximately spherical and the particles of the cover layer are
aspherical.

15. A method of making a medical implant device, comprising: a. forming a
substrate having an undulating surface provided by peaks which are
separated by recesses, b. applying a layer of particles on the said
surface of the substrate, wherein the spacing between adjacent peaks on
the surface of the substrate is less than the particle size of the
particles, and c. bonding (i) particles to the peaks on the surface of
the substrate and (ii) adjacent particles to one another.

Description:

[0001] This invention relates to a medical implant device having a porous
surface for tissue ingrowth.

[0002] Medical implant devices can be anchored at an implant site by
ingrowth of tissue into a porous surface region of the implant. For
example, U.S. Pat. No. 3,855,638 discloses a component of an orthopaedic
joint prosthesis which comprises a metal substrate having a porous metal
coating into which bone tissue can grow. The coating is provided by metal
particles which are jointed to each other and to the substrate to define
a plurality of connected, interstitial pores which are distributed
throughout the coating. The coating is formed by a sintering process.

[0003] EP-A-1997524 discloses a component of an orthopaedic joint
prosthesis in which a substrate has a porous surface region provided by
two layers of metal particles which are bonded to one another and to the
surface of the substrate by a sintering process. The inner layer
comprises spherical particles and the outer surface comprises aspherical
particles. The layer of aspherical particles results in increased
roughness of the surface of the component, compared with a component
whose surface is provided by spherical particles. The porosity of a
surface layer provided by aspherical particles can increase progressively
towards the surface of the component, and the porosity of the layer can
be greater at the surface than the porosity of a layer which is formed
from spherical particles with similar size. It is therefore possible to
accommodate greater ingrowth of bone tissue, which provides for stronger
fixation of the component when implanted.

[0004] U.S. Pat. No. 5,443,510 discloses an implant in which a porous
surface region is created by sintering beads on to a mesh which is welded
to the implant surface. This is said to reduce notch formation at sinter
sites on the substrate surface of a device in which beads are sintered
directly on to the surface. The mesh increases the thickness of the
surface region of the implant. The security of the fixation of the
implant depends in part on the fixation of the mesh to the substrate
surface.

[0005] The present invention provides an implant device which has an
undulating surface provided by peaks which are separated by recesses,
with particles bonded to the undulating surface which are bigger than the
gaps between the peaks.

[0006] Accordingly, in one aspect, the invention provides a medical
implant device, which comprises a substrate having an undulating surface
provided by peaks which are separated by recesses, in which the device
includes a porous coating layer provided on the undulating surface of the
substrate which comprises a plurality of particles, in which the spacing
between adjacent peaks on the surface of the substrate is less than the
particle size of the particles, and in which particles are bonded to the
peaks on the surface of the substrate and adjacent particles are bonded
to one another.

[0007] In another aspect, the invention provides a method of making a
medical implant device, which comprises: [0008] a. forming a substrate
having an undulating surface provided by peaks which are separated by
recesses, [0009] b. applying a layer of particles on the said surface of
the substrate, in which the spacing between adjacent peaks on the surface
of the substrate is less than the particle size of the particles, and
[0010] c. bonding (i) particles to the peaks on the surface of the
substrate and (ii) adjacent particles are bonded to one another.

[0011] The particles and the shapes of the peaks and the recesses between
them should be such that the particles cannot fit into the recesses to
contact the surface of the substrate at the bases of the recesses. Bonds
between the particles and the substrate surface are therefore formed on
the peaks (on the tops of the peaks or on the sides of the peaks), spaced
apart from the base of the recesses.

[0012] The implant of the present invention has the advantage that the
tendency for notches to initiate and to propagate through the substrate
is reduced because the particles are bonded to the peaks on the substrate
surface. In the event that a notch starts to form in a peak at the
interface between the peak and a particle, the notch can propagate
through the peak as far as an adjacent recess. Further propagation can
then be inhibited, in particular through the bulk of the implant
substrate.

[0013] Preferably, the surfaces of a particle and the top or side of a
peak to which the particle is bonded are both convex. When the bond is
formed by sintering, the extent of deformation of the surface of the
substrate can be reduced compared with the surface of a substrate which
is essentially planar (without an array of peaks and recesses). It is
believed that this reduced deformation of the surface can help to reduce
the initiation of notches in the substrate.

[0014] The device of the invention is believed to have an increased
resistance to fatigue failure by virtue of the provision of peaks and
recesses on the surface of the substrate.

[0015] In particular, the implant of the invention can have the advantage
of improved fatigue strength compared with a device in which the
substrate does not have the peaks and recesses feature of the present
invention.

[0016] Preferably, the peaks are spaced apart regularly on the surface of
the substrate. This can facilitate manufacture of the substrate. It can
help to ensure that particles whose size is within an appropriately
controlled range contact the peaks on the substrate surface and do not
sit in the recesses between the peaks.

[0017] The peaks can comprise ridges in which the length of each ridge is
greater than its width, and in which the recesses comprise troughs
between the ridges. For example, the substrate surface can be provided by
a plurality of approximately uniformly spaced ridges having troughs
between them. A plurality of uniformly spaced ridges having troughs
between them can be formed as a helical thread.

[0018] The recesses can comprise troughs in which at least some of the
peaks, preferably all of the peaks, are surrounded by troughs when viewed
in plan from above the surface. The peaks can be rounded, for example
circular, at least at the top of the peaks, when viewed from above the
surface. The peaks can have several sides. For example the peaks might
have at least four sides. The peaks can have four sides when the troughs
are straight and are in two intersecting arrays with the angle between
two arrays of troughs is 90°. The peaks will be square when the
distances between the troughs of the first array is equal to the distance
between the troughs of the second array. The peaks might have three sides
when the troughs are straight and are in three intersecting arrays, for
example with the angle between the troughs of the arrays being
60°.

[0019] Preferably, the base of a recess between adjacent peaks is rounded
when viewed in cross-section. This can help to reduce the tendency for
cracks to form at the bases of the recesses. Preferably, the portions of
the bases of the recesses that are rounded have an approximately constant
radius. The constant radius portions of the bases of the recesses can
extend through an angle of arc of at least about 45°, preferably
at least about 60°, more preferably at least about 80°,
especially at least about 100°, for example at least about
120° or at least about 130°. Preferably, the constant
radius portions of the bases of the recesses extend through an angle of
arc of not more than about 175°, for example not more than about
160° or not more than about 150°.

[0020] Preferably, the ratio of the transverse dimension of the particles
(which will be their diameter when they are spherical) to the radius of
the recesses is at least about 1.0, more preferably at least about 1.1,
especially at least about 1.2, for example at least about 1.25. This can
help to ensure that the particles cannot fit into the recesses to contact
the surface of the substrate at the bases of the recesses.

[0021] Preferably, the peaks are rounded when viewed in cross-section.
This can help to reduce the tendency for any cracks to initiate at the
interface between a peak and a particle which is bonded to it.
Preferably, the portions of the peaks that are rounded have an
approximately constant radius. The constant radius portions of the bases
of the recesses can extend through an angle of arc of at least about
45°, preferably at least about 60°, more preferably at
least about 80°, especially at least about 100°, for
example at least about 120° or at least about 130°.
Preferably, the constant radius portions of the peaks extend through an
angle of arc of not more than about 175°, for example not more
than about 160° or not more than about 150°.

[0022] Preferably the ratio of the radius which defines the base of a
recess which is rounded to the ratio of the radius which defines an
adjacent peak which is rounded is at least about 0.7, more preferably at
least about 0.8, especially at least about 0.9. Preferably the said ratio
is not more than about 1.3, more preferably not more than about 1.2,
especially not more than about 1.1.

[0023] Preferably the ratio of the distance between adjacent recesses
which are rounded to the radius which defines the base of the recesses is
at least about 2.5, more preferably at least about 3.5. The value of the
ratio will generally be not more than about 6, preferably not more than
about 5.

[0024] Preferably the ratio of the distance between adjacent peaks which
are rounded to the radius which defines the peaks is at least about 2.5,
more preferably at least about 3.5. The value of the ratio will generally
be not more than about 6, preferably not more than about 5.

[0025] Preferably the ratio of the height of two peaks which are rounded,
measured from the base of the recess between them, to the radius which
defines the peaks is at least about 1.0, more preferably at least about
1.2. Preferably, the value of the ratio is not more than about 2.0, more
preferably not more than about 1.6, for example not more than about 1.4.

[0026] Preferably, the ratio of the radius of the peaks to the size of the
particles which are bonded to the peaks is not more than about 0.7, more
preferably not more than about 0.6. Preferably, the ratio of the radius
of the peaks to the size of the particles which are bonded to the peaks
is at least about 0.3, more preferably at least about 0.4.

[0027] Preferably the ratio of the height of the peaks, measured from the
base of the recess between them which is rounded, to the radius which
defines the recess is at least about 1.0, more preferably at least about
1.2. Preferably, the value of the ratio is not more than about 2.0, more
preferably not more than about 1.6, for example not more than about 1.4.

[0028] Preferably, the particles which are bonded to the peaks on the
surface of the substrate have a generally rounded shape so that they do
not have any edges or corners. Preferably, the particles which are bonded
to the peaks on the surface of the substrate are approximately spherical
so that, prior to any changes in shape resulting from the process of
bonding them to the substrate, the surface of any one of particles is
defined by an approximately constant radius (meaning that the longest
chord measured across the particle varies by not more than 10%).

[0029] The sizes of the particles which are used in the device of the
invention can be measured using mesh sieves. Particles which are applied
to the surface of the substrate can have a single mode particle size
distribution or a multimode (for example bimodal) particle size
distribution. The device can have applied to it a first layer of
particles which have a first size distribution and a second layer of
particles which has a second size distribution.

[0030] Preferably the particle size of the particles which are bonded to
the peaks on the surface of the substrate is at least about 50 μm,
more preferably at least about 80 μm, especially at least about 120
μm, for example at least about 150 μm. The size of the spherical
particles will generally be not more than about 400 μm, preferably not
more than about 325 μm, more preferably not more than about 275 μm,
for example not more than about 250 μm. There will generally be a
spread of particle sizes; it will generally be preferred that at least
85% by weight of the particles meet these size limitations.

[0031] Preferably, the distance between two adjacent peaks on the surface
of the substrate is at least about 50 μm, more preferably at least
about 80 μm, especially at least about 120 μm, for example at least
about 150 μm. Preferably, the distance between two adjacent peaks is
not more than about 400 μm, more preferably not more than about 320
μm, especially not more than about 270 μm, for example not more
than about 240 μm.

[0032] Preferably, the size profile of the particles which are bonded to
the surface of the substrate is such that they are able to pack together
in an approximately close packed array. This is facilitated by minimising
the spread of the sizes of the particles. Preferably, the peaks on the
surface of the substrate are able to fit in to the close packed array of
particles which are bonded to it so that the layer of the particles which
is in immediate contact with the surface is approximately close packed.

[0033] Preferably the ratio of the particle size of the particles which
are bonded to the peaks on the surface of the substrate to the distance
between two closest adjacent peaks is at least about 0.8, more preferably
at least about 0.9. Preferably the ratio of the particle size of the
particles which are bonded to the peaks on the surface of the substrate
to the distance between two closest adjacent peaks is not more than about
1.2, more preferably not more than about 1.1, for example about 1.0.

[0034] Preferably, the device includes a cover layer which overlies the
coating layer and which is provided by a plurality of cover layer
particles, in which the cover layer particles are bonded to the particles
of the coating layer and adjacent particles of the cover layer are bonded
to one another, and in which the particles of the coating layer are
approximately spherical and the particles of the cover layer are
aspherical.

[0035] The aspherical particles can be rounded in shape. Preferably, the
shape of the aspherical particles is angular so that it is defined by
corners and/or edges and/or recesses. The size of aspherical particles
can be established using mesh sieves so that the measured particle size
is defined by the size of the apertures in a sieve through which the
particles can pass. One or more layers of aspherical particles can be
applied on top of one or more layers of particles which are spherical.

[0036] Preferably, the particles on the surface of the substrate are
provided in one or more layers which have a total thickness, measured
from the top of the peaks on the substrate surface, of at least about 250
μm, more preferably at least about 300 μm, especially at least
about 500 μm. The thickness can be at least about 800 pm or at least
about 1000 μm. The thickness of the layers of particles on the surface
of the substrate will normally be not more than about 2000 μm.

[0037] The surface of the substrate, at least towards the surface on which
the particles are provided, will generally be provided by a metal. The
particles will generally be formed from a metal. It will generally be
preferred that the material of the substrate, at least at its surface, is
substantially the same as the material of the particles. Metals which
might be used in the implant device include titanium and its alloys (such
as, for example, Ti-6Al-4V), tantalum and its alloys, stainless steels
such as are commonly used in the manufacture of implant devices, and
alloys of cobalt and chromium (optionally with other elements such as for
example molybdenum). When the surface of the substrate and the particles
are both provided by a metal, it can be preferred to bond the particles
to the substrate and to each other by a sintering process. The particles
can be suspended in a slurry in an aqueous solution which contains an
organic binder such as, for example, methyl cellulose. The slurry can be
held in a mould, surrounding the portion of the surface of the substrate
which is to have the particles applied to it. The slurry is heated to
remove the water. The particles and the substrate are then exposed to
heat in an inert or reducing atmosphere to burn off the binder and to
fuse the particles to one another and to the substrate.

[0038] A suitable sintering process can involve: [0039] Ensuring that
the substrate surface is clean and smooth. [0040] Applying a coating of
organic binder to the substrate surface. [0041] Submerging the substrate
in a fluidised bed of metal particles for 2 to 3 seconds while agitated.
[0042] Applying an overspray coating of binder. [0043] Repeating the
submersion and overspray steps. [0044] Removing any high spots; filling
any low spots. [0045] Repeating the coating, submersion and overspray
steps as necessary to build up a sufficient thickness of particles.
[0046] Placing the coated product in an oven.

[0047] An implant device in which the substrate is formed from titanium
and the particles are formed from titanium can be sintered adequately at
a temperature of about 1250° C. The length of the period in which
the device is exposed to elevated temperature will depend on the mass of
the substrate. The period must be sufficiently long for the particles to
be fixed securely to the substrate. By way of example, in the case of the
acetabular shell component of a hip joint prosthesis, a heating time of
from 100 to about 300 minutes will generally be appropriate. Longer times
can be appropriate for articles in which the substrate has a greater
thermal mass.

[0048] The technique which is used to create the peaks and recesses
features will depend on the shape and arrangement of the features on the
surface of the substrate. It will generally be preferred that the
features are formed by a machining process. The creation of recesses
which are in the form of relatively long troughs which separate adjacent
peaks can be facilitated by forming them as a continuous helical thread.
Relatively long troughs, whether in the form of discrete circular
features or a continuous helical thread, can be created on the surface of
a substrate having a circular cross-section using a lathe. Peaks and
recesses features can be formed on substrates having other shapes and in
other forms using different techniques such as etching techniques, for
example acid etching or laser etching or a combination of the two.

[0049] The bone facing surface of the implant device can be treated to
promote desirable interactions with bone tissue, for example to encourage
ingrowth of bone tissue. For example, the surface of the device can be
coated with a hydroxy apatite material.

[0050] Embodiments of the invention are described below by way of example
with reference to the accompanying drawings, in which:

[0051] FIG. 1 is a side view of a femoral component of a hip joint
prosthesis, which includes particles bonded to the surface of a
substrate.

[0052]FIG. 2 is a schematic cross-sectional view of the surface region of
the femoral component shown in FIG. 1.

[0053]FIG. 3 is an isometric view of the surface of a substrate whose
surface is provided by a plurality of peaks which are separated by
troughs, in which each peaks is surrounded by the troughs.

[0054] FIG. 1 shows a femoral component of a hip joint prosthesis which
includes a stem 2 which can be fitted into the prepared intramedullary
cavity in the femur, and a head 4 which can articulate in the hollow
acetabular component of the prosthesis. The stem has a proximal
epiphyseal portion 6 and a distal portion 8. The stem is formed from
titanium to a conventional shape and using techniques which are well
known from the manufacture of orthopaedic joint prostheses.

[0055] The epiphyseal portion 6 of the stem 4 includes a layer of titanium
particles which have been bonded to the surface of the stem substrate by
means of a sintering process.

[0056] While FIG. 1 shows the application of the invention to the femoral
component of a hip joint prosthesis, the invention can be applied to the
bone facing surface of other implant components where it is intended that
bone tissue should bond to the surface of the components by ingrowth of
bone tissue, including for example the acetabular component of a hip
joint prosthesis, the femoral component of a knee joint prosthesis, the
tibial component of a knee joint prosthesis, the humeral component of a
shoulder joint prosthesis, the glenoid component of a shoulder joint
prosthesis, components of elbow and ankle joint prostheses, components
such as pins, nails, and rods (including intramedullary pins, nails and
rods) and plates such as might be used in the treatment of fractures, and
components for implantation in a patient's spine such as rods, plates
etc.

[0057]FIG. 2 shows the stem substrate 10 which has a helical groove
formed on its surface which defines a plurality of peaks 12, separated by
grooves 14. The top of each peak is rounded with a radius of 50 μm.
The bottom of each groove is rounded with a radius of 50 μm. The
spacing between two adjacent grooves is 200 μm. The depth of each
groove is 80 μm.

[0058] The titanium particles 16 which are bonded to the surface of the
epiphyseal portion of the stem substrate are spherical with a radius of
about 100 μm. The particles are sized so that sit against the rounded
surfaces of the peaks 12 and so that they cannot fit into the grooves 14
between the peaks 12 to contact the bases of the grooves. Successive
layers of particles can form close packed array with the particles which
sit against the rounded surfaces of the peaks. It should be appreciated
that the arrangement of the particles on the surface of the substrate as
shown in FIG. 2 is schematic. It might be that peaks on the surface of a
substrate might not have particles bonded to them. It will be understood
however that the size of the particles and the shapes of the peaks and
the recesses between them are such that the particles cannot fit into the
recesses to contact the surface of the substrate at the bases of the
recesses.

[0059] The implant device can have a cover layer of aspherical particles
applied to the spherical particles 16, for example as disclosed in
EP-A-1997524.

[0060]FIG. 3 shows a substrate 30 whose surface is provided by a
plurality of peaks 32. Each peak is surrounded by a recess in the form of
a trough. The depth of the troughs in the embodiment shown in FIG. 3
varies around the periphery of the peaks. However it is envisaged that
the depth of the troughs might be approximately constant around the
peaks. Particles can be bonded to the surface of the substrate 30 so that
they contact one or more of the peaks. Appropriately sized particles will
sit against the rounded surfaces of the peaks and so that they cannot fit
into the troughs between the peaks to contact the bases of the troughs.
The particles will generally be sized so that they contact a square array
of four peaks.